Magneto-inductive flow measuring devices make use of the principle of electrodynamic induction for measuring volumetric flow. According to such principle, charge carriers of the medium moved perpendicularly to a magnetic field induce a voltage in measuring electrodes arranged essentially perpendicular to the flow direction of the medium and perpendicular to the magnetic field. The measuring electrodes are coupled with the medium either galvanically or capacitively. This voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the tube; it is, thus, proportional to the volume flow rate.
Usually, such flow measuring devices are integrated into a processing installation. The flow of a medium through a processing installation arises, frequently, not naturally, e.g. because of falling, but, instead, is produced by a pump. Pumps are usually driven by an electric motor. Just as a varying magnetic field is necessary for operating a magneto-inductive flow measuring arrangement, so also does the rotor of an electric motor rotate in a timewise-varying magnetic field. This circumstance is made use of in the apparatus of the present invention.